A vacuum countergravity casting process using a gas permeable mold is described in such prior patents as the Chandley et al U.S. Pat. Nos., 4,340,108 issued July 20, 1982 and 4,606,396 issued Aug. 19, 1986. That countergravity casting process involves providing a mold having an expendable porous, gas permeable upper mold member (cope) and an expendable lower mold member (drag) engaged together at a parting plane, sealing the bottom lip of a peripheral wall of a vacuum chamber to the mold such that the vacuum chamber confronts the gas permeable upper mold member, submerging the bottom side of the lower mold member in an underlying molten metal pool and evacuating the chamber to draw the molten metal through one or more ingate passages in the lower mold member and into one or more mold cavities formed between the upper and lower mold members.
Typically, this process fills the mold cavities with molten metal in a very short time (approximately 2-3 seconds). However, the mold must remain submerged in the molten metal pool until the molten metal in the ingate passages of the lower mold member freezes off (solidifies). For example, the mold typically must remain submerged in the molten metal pool on the order of 15 to 50 seconds after mold filling to freeze (solidify) the molten metal in the ingate passages. In the event the mold is raised out of the molten metal pool prior to freezing of the molten metal in the ingate passages, the molten metal in the ingate passages as well as in the mold cavities can flow downwardly out of the mold and result in a defective casting which must be scrapped.
Moreover, such expendable counterqravity casting molds are complex in that in addition to the mold cavities themselves, they also require vacuum chamber sealing surfaces thereon as well as means for securing the molds to the vacuum chamber. They are typically relatively expensive (e.g., compared to green sand molds) in that they require more expensive materials (e.g., resin-sand mixtures and curing agents therefor).
A technique for permitting use of less complex molds made from lower quantities of, and perchance cheaper, mold materials is illustrated in copending U.S. patent application Ser. No. 238,724 entitled Countergravity Casting Apparatus And Process For Casting Thin-Walled Parts wherein one or more casting molds are positioned on a reuseable bottom drag slab that is immersed in the molten metal supply pool to draw molten metal upwardly through an ingate in the drag slab and into the casting mold(s) thereon. Although the ingate of the drag slab may include a molten metal filter therein for removing inclusion-forming impurities from the molten metal as it is drawn upwardly into the casting molds, there is no means in the ingate to prevent back flow of unsolidified metal from the casting mold in the event the drag slab is removed from the pool too soon; i.e., before the molten metal in the ingate and mold cavity solidifies.
It is an object of the present invention to provide an improved, cost effective casting apparatus and method for the vacuum-induced, countergravity casting of molten metal characterized by significantly reduced immersion times of a bottom drag member of the apparatus in the underlying molten metal supply pool as a result of valve means being positioned in the ingate of the drag member to preclude back flow of molten metal when the drag member is removed from the pool prior to solidification of the molten metal cast into the apparatus. It is another object of the present invention to position the valve means in the ingate adjacent a parting plane defined between the bottom drag member and a casting mold member disposed on the bottom drag member to facilitate manufacture and assembly and such as to permit the valve means (i.e., a valve member) to move between a valve seat on the bottom drag member and a stop surface on the casting mold member. These and other objects and advantages of the present invention will become more readily apparent from the description thereof which follows.